This application is based upon and claims priority from Japanese Patent Applications No. 8-211086 filed Aug. 9, 1996, No. 8-211088 filed Aug. 9, 1996, No. 8-211089 filed Aug. 9, 1996, No. 8-230731 filed Aug. 30, 1996, No. 8-230732 filed Aug. 30, 1997, and No. 9-86331 filed Apr. 4, 1997, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a semiconductor acceleration sensor for detecting acceleration by utilizing a semiconductor material having a large piezoresistance coefficient, particularly to a semiconductor acceleration sensor that is constituted to be able to detect acceleration at a comparatively low level of substantially xc2x11 G or smaller.
2. Description of Related Art
There is a semiconductor acceleration sensor that is formed into a shape where a weight portion dislocated by receiving acceleration is supported by an outer frame via beams at which diffused resistors are formed, by etching a silicon substrate. According to this sensor, when acceleration is applied thereon, the weight portion is dislocated by receiving a force proportional to the acceleration and therefore, the beams supporting the weight portion are distorted by which resistance values of the diffused resistors are changed through a piezoresistance effect in response to stresses caused by the distortion. The change of the resistance value can be detected as a voltage signal by forming a detecting circuit where the diffused resistors are connected in a bridge connection and the applied acceleration can be detected based on the voltage signal.
Meanwhile, such a semiconductor acceleration sensor is used for detecting, for example, a vehement impact that occurs in a case of an automobile collide. Because this semiconductor acceleration sensor detects the degree of impact received by a detected portion as a magnitude of acceleration, acceleration to be detected has a large acceleration value which exceeds 10 G (G is a gravitational acceleration of 9.8 m/sec2).
Meanwhile, it has been requested in recent years for an automobile or the like to achieve promotion of safety by carrying out braking control or the like by detecting a very small acceleration to a degree of level of acceleration or deceleration caused in a normal running state, which is far smaller than the acceleration caused by impact as described above. Accordingly, an acceleration sensor capable of accurately detecting an acceleration having a range from substantially xc2x11 G to at least substantially xc2x12 G as a range of acceleration to be detected has been requested.
However, such a semiconductor acceleration sensor for detecting a very small acceleration has the following technological problem. That is, a very small force which the weight portion of the sensor chip receives by acceleration, is caused as strain of the beams and accordingly, when the beams receives a force even slightly through the frame portion to which a sensor chip per se is fixed, stresses are caused in the beams supporting the weight portion and the diffused resistors on the beams are applied with stresses by which adverse influence may be effected on the detecting operation, such as variation of the sensitivity of detection.
In order to deal with such a drawback, according to a semiconductor acceleration sensor having a small detection range of from xc2x11 G to xc2x12 G as the detection range of acceleration, a structure for reducing as less as possible stresses received from a substrate, is adopted and an outline of the structure is shown, for example, in FIG. 44 and FIG. 45. That is, a semiconductor sensor chip 1 made of silicon is formed in a state where a sensor element 3 is supported by a cantilever 4 in a first frame 2 which constitutes an outer frame.
The sensor element 3 is constituted by a second frame in an U-like shape, four beams 6a through 6d extended from the second frame 5 and a weight portion 7 supported by the four beams 6a through 6d. Diffused resistors are previously formed at the four beams 6a through 6d where resistances thereof are varied by the piezoresistance effect when they receive distortion. Further, the diffused resistors are wired in a bridge connection whereby a change in the resistance can be outputted as a voltage signal.
According to the semiconductor sensor chip 1, the first frame 2 is fixed to a seat 8 made of glass by anodic bonding. A recess 8a is formed in the seat 8 on a side thereof more inner than a portion thereof facing to the first frame 2 by which even if the weight portion 7 is deformed it is not brought into contact with the recess 8a. The seat 8 made of glass is bonded and fixed to a substrate 9 made of ceramic. An IC chip 10 for carrying out signal processing of an output from the sensor is attached to the substrate 9 by die bonding and the IC chip 10 and the semiconductor sensor chip 1 are electrically connected by a bonding wire 11.
The substrate 9 to which the semiconductor sensor chip 1 is fixedly adhered via the seat 8, is arranged in a case 12 comprising a base 12a and a cap 12b. Oil 13 is filled in the case 12 as a damper material for preventing the device from destructing when an excessive acceleration is applied thereon. A lead, not shown, electrically connected to the semiconductor sensor chip 1 or the IC chip 10 is extended from the case 12 to outside by which a detection signal is outputted.
According to the above-described constitution, when the semiconductor sensor chip 1 receives an acceleration orthogonal to a face thereof, the weight portion 7 is dislocated in a direction opposed to the acceleration by a force at that moment, whereby the diffused resistors formed at the beam 6a through 6d are applied with a distortion in accordance with the acceleration. Then, an output voltage of a circuit in a bridge connection is varied by the piezoresistance effect of the diffused resistors and accordingly, the applied acceleration can be detected.
However, by adopting such a structure, the portion of the first frame 2 needs to be constituted extraneously in addition to essential portions for detecting acceleration according to the semiconductor acceleration sensor 1 and therefore, it is inevitable to increase the chip size by the first frame 2. As a result, the portion of the first frame 2 becomes a hindrance for downsizing a total size of the sensor.
Further, according to the above-described sensor, a seal mechanism for preventing leakage of the oil 13 is needed and the like, which gives rise to general complication of the structure.
Moreover, there has been in recent years an increase in needs for detecting acceleration at a comparatively low level of substantially 1 G or lower in the usage of ABS (Antilock Braking System) or a device for preventing transverse skidding in curving operation of an automobile, however, according to the conventional acceleration sensor utilizing oil damping as mentioned above, it is difficult to sufficiently lower the detectable acceleration.
That is, the conventional sensor has a drawback where stable detecting operation may not be carried out due to the oil 13 filled in the case 12 as a damper material whereby the temperature range in use may be limited or detection error may be enlarged.
For example, according to a result of actual measurement of a degree of varying sensitivity (which indicates as a value of percentage a degree of varying sensitivity in a case where the sensitivity is defined as a value of a difference between output voltages when acceleration is 0 G and when it is 1 G) in the above-described structure in the case where the temperature range for use is as wide as from substantially xe2x88x9230xc2x0 C. to 85xc2x0 C., a dispersion of about xe2x88x922.5% as a minimum value and about ""1% as a maximum value is caused. Therefore, since the degree of varying sensitivity that is practically necessary in accurately measuring acceleration of substantially xc2x11 G in the above-described temperature range for use, is about 1 through 2% and therefore, there causes a case where such a condition cannot be satisfied.
Next, an investigation has been performed on a semiconductor acceleration sensor having a structure capable of responding to the needs for detecting acceleration at a comparatively low level of substantially xc2x11 G or lower after resolving the above-described problems once for all by making free use of micro machining technology.
Specifically., a semiconductor sensor chip is mounted on a seat made by a material having a thermal expansion coefficient equivalent to that of the material of the chip (preferably the same material), whereby adverse influence caused by a distortion due to a difference in the thermal expansion coefficients of both is eliminated. Further, air damping for a weight portion is carried out by an air gap formed between the weight portion and the seat, whereby simplification of structure is realized by dispensing with the oil 13 and a seal structure thereof as shown in FIG. 45 and further, the acceleration at a comparatively low level of substantially xc2x11 G or lower can be detected by enhancing an accuracy in fabricating beams and the like.
When a semiconductor acceleration sensor having such a structure was actually fabricated, and an output characteristic thereof, particularly an output value in the state of 0 acceleration for constituting the reference of the sensor characteristic (hereinafter, referred to as 0 G output) was measured, a phenomenon where the 0 G output was dispersed at every time of measurement, was caused. The inventors carefully repeated diversified experiments and analyses of results of the experiments with regard to such a phenomenon and reached a conclusion that the phenomenon was caused by electrostatic attraction caused at the inside of the semiconductor acceleration sensor.
That is, it was ascertained as follows. The semiconductor sensor chip was applied with power source voltage for driving it and therefore, it was inevitable that an electrostatic capacitance to some degree was present between the semiconductor sensor chip and the seat. As a result, an electrostatic induction phenomenon where electric charges having different polarities respectively gathered at the surface of the weight portion and the surface of the seat opposing to the weight portion via a predetermined air gap, was caused. An electrostatic attraction was operated between the weight portion and the seat by influence of an electric field caused by the electrostatic induction phenomenon, whereby the dimension of the air gap therebetween was varied from an initial set value and such a variation caused the dispersion in the 0 G output. It was ascertained further that there was a phenomenon where a width of the variation of the 0 G output differed in accordance with the initial set value of the air gap dimension and a potential difference caused between the semiconductor sensor chip and the seat.
Further, the detection range of acceleration is in a very small region of substantially xc2x11 G through xc2x12 G and therefore, it is necessary to thinly form the thickness dimension of the beams to improve a detection sensitivity in the structure of the semiconductor sensor chip, however, the detection sensitivity was not necessarily improved only by forming thinly the thickness dimension thereof.
The present invention has been made in view of the above-described situation and it is a first object of the present invention to provide a semiconductor acceleration sensor capable of obtaining stable output characteristic only by adopting a constitution for controlling a dimension of a gap between a weight portion dislocating in accordance with acceleration and a seat opposed thereto or a constitution for restraining a potential difference caused between the semiconductor sensor chip and the seat, and capable of realizing simplification of structure.
It is a second object of the present invention to provide a semiconductor acceleration sensor capable of preventing as less as possible adverse influence caused by stresses from the side of a substrate thereby achieving downsizing thereof depending on detection level even with a constitution whereby acceleration that is comparatively small to a degree of the acceleration of xc2x11 G through xc2x12 G, can accurately be detected.
Further, it is a third object of the present invention to provide a semiconductor acceleration sensor capable of carrying out high accuracy detection operation even in the case where detection range is, for example, substantially xc2x11 G as a further smaller acceleration range and temperature range for use is wide.
To attain the first object, a semiconductor acceleration sensor according to the present invention includes a semiconductor sensor chip having a weight portion supported via beams, for detecting an acceleration up to substantially xc2x11 G by utilizing a piezoresistance effect of resistor elements formed on the beams. The semiconductor sensor chip is supported by a seat formed by a material having a thermal expansion coefficient equivalent to a thermal expansion coefficient of the semiconductor sensor chip and the weight portion is arranged at a vicinity of the seat whereby an air damping operation of the weight portion is carried out. A dimension of an air gap between the weight portion and the seat is set to 7 xcexcm or more.
The inventors have ascertained from their experiments that if the dimension of the air gap between the weight portion and the seat is 7 xcexcm or more, an variation of the 0 G output caused by the electrostatic attraction generating between the weight portion and the seat falls within an allowable range.
Therefore, according to the semiconductor acceleration sensor of the present invention, deterioration of the output characteristics thereof caused by the electrostatic attraction generated in the interior of the sensor can be prevented, thereby obtaining stable output characteristics. Further, because the oil for damping use can be omitted, simplification of the structure can be achieved. Moreover, because the thermal expansion coefficient of the semiconductor acceleration sensor chip is equivalent to that of the seat supporting the sensor chip, distortion generating between the semiconductor sensor chip and the seat can be suppressed.
To attain the second object, the semiconductor acceleration sensor of the present invention includes a semiconductor sensor element having a weight portion supported via beams, for detecting an acceleration up to substantially xc2x11 G through xc2x12 G by utilizing a piezoresistance effect of resistor elements formed on the beams. A thickness dimension of the beams in the semiconductor sensor element is set to be equal to or more than a dimension when a variation amount of a sensitivity in a temperature range for using the semiconductor sensor element becomes a predetermined allowable value.
The inventors gave attention to the thickness dimension of the beams of the semiconductor acceleration sensor element, to improve detection accuracy of the semiconductor acceleration sensor which detects a very small acceleration in a range of xc2x11 G through xc2x12 G. As long as the beams are formed to satisfy the condition of that dimension, detecting operation of high accuracy can be performed. Further, downsizing of the sensor can be also achieved.
To attain the third object, the semiconductor acceleration sensor of the present invention includes a semiconductor sensor element having a weight portion supported via beams, for detecting an acceleration up to substantially xc2x11 G by utilizing a piezoresistance effect of resistor elements formed on the beams. The semiconductor sensor element is supported by a seat, formed by a material having a thermal expansion coefficient equivalent to a thermal expansion coefficient of the semiconductor sensor element and arranged at a vicinity of the weight portion thereby carrying out an air damping operation. The semiconductor sensor element further comprises an inner frame for supporting the weight portion via the beams and an outer frame fixed to the seat, for supporting the inner frame via a thick connecting portion. A thickness dimension of the beams is set to be equal to or more than a dimension when a variation amount of a sensitivity in a temperature range for using the semiconductor sensor element becomes a predetermined allowable value.
According to this semiconductor acceleration sensor, to detect with high accuracy a further small acceleration in a range up to substantially xc2x11 G, the thickness dimension of the beams is controlled to fall in a desired range, the a frame portion for supporting the weight portion is composed of a double frames of the inner frame and the outer frame, and moreover, air damping is carried out.
Therefore, when the weight portion is dislocated by a force receiving from acceleration acting on the semiconductor sensor element, the beams supporting the weight portion from the inner frame is distorted, whereby the resistances of the resistor elements are changed and therefore, the acceleration can be detected. At this time, because the inner frame is fixed to the seat in a state of being supported by the outer frame through the thick connecting portion, even if stresses extend from the seat to the frame portion, for example, by variation in an environmental temperature, it can be prevented as much as possible for the stresses to extend to the inner frame. Moreover, because an air damping structure is adopted, an adverse effect caused by temperature variation, which is received through the damping material such as an oil, can be eliminated. Therefore, the semiconductor acceleration sensor can detect with high accuracy a further small acceleration of substantially xc2x11 G in a wide temperature range for use.